1. Field of the Invention
The present invention relates to an operational amplifier, and more specifically to an operational amplifier for use in a liquid crystal display (LCD) source driver, for instance.
2. Description of the Background Art
Generally, a liquid crystal display source driver has a large number of output operational amplifiers. With the recent tendency towards the large-sized liquid crystal display, it has been desired to improve various characteristics of liquid crystal driving devices. Under those circumstances, the number of the data and scanning lines is accordingly increasing and, in particular, it is desired to elevate the writing speed to liquid crystal capacitances for the scanning lines. Moreover, with increasing competition in the market of liquid crystal display devices, the cost of component parts is necessarily required to decrease. The same is true for driving devices, which are desired to be inexpensive with higher driving performance.
As the technique that possibly satisfies the above requirements, there is a technique disclosed in Japanese patent laid-open Publication No. 164263/1994. This Publication discloses an operational amplifier which accomplishes a high slew rate with its phase margin maintained higher. FIG. 9 is a schematic circuit diagram for simplified illustration of an operational amplifier described in the publication, and FIG. 10 is a timing chart for use in understanding the operation of the operational amplifier shown in FIG. 9. The operational amplifier has switches SW1 and SW2 and a phase compensator capacitance C1. The switch SW1 is turned on responsive to switching of a signal TP1, and at the same time the switch SW2 is turned off responsive to a signal CTRL delivered from outside the chip of the operational amplifier. During the time the phase compensator capacitance C1 is disconnected from an output OUT of the operational amplifier, the output potential is changed acutely. When the signal CTRL is again changed, the switch SW2 is turned on, thereby connecting the phase compensator capacitance C1 to the output. With the above constitution and operation, it is possible to provide for a high slew rate with the phase margin satisfactory.
However, with the above operational amplifier, it is necessary to provide the switch to control the connection between the phase compensator capacitance and the output, and in order to control the switch it is also necessary to deliver the control signal from outside. This control signal needs to be delivered from, for example, a timing controller (T-CON). In order to let the timing controller deliver the control signal, the circuit must be designed with an attention paid to how long the switch is to be turned on or off, thus raising the costs involved in the timing controller and ascribable to the protracted development time period. Since the phase compensator capacitance is kept in its off state for a certain period of time by the control signal and the switch, it is true that the slew rate is improved indeed whereas there is a problem that stability against oscillation as well as continuity in the output waveform may be lost until the phase compensator capacitance is turned on.